This invention relates to roller table heat shields for metal hot rolling mills.
In the hot rolling of steel in slab form, when a transfer bar passes along a roller table between roughing mill and finishing mill, a typical journey time might be about 130 seconds. In the absence of any means to reduce heat loss the temperature of the bar drops during this period. A thin transfer bar (up to about 25 mm thick) undergoes an edge temperature loss similar to the temperature loss at the centre of its width. However, with thicker transfer bars (about 40 mm or more thick) the edge of the material undergoes a greater temperature loss than the centre of the width because a significant amount of heat is being radiated from the side edge in addition to radiation from the top and bottom surfaces. The edge temperature loss can be so great as to affect significantly the metallurgical properties and edge cracking can occur during finish rolling.
Because of this problem it is known to install induction edge heaters to reheat the edges of transfer bars prior to entry to the finishing mill. However, this is wasteful: the power input required can be as much as 5 MW. Moreover, because the width of the transfer bars can vary considerably from one to the next, means have to be provided for moving the edge heaters to position them close to the edges of each bar.
It is also known to provide heat insulating means around the hot material path along a roller table to reduce temperature loss from hot material in slab or strip form, as illustrated in EP 005340, for example. Upper, lower and side heat insulating panels arranged as a closed, high efficiency, heat shield may be able to retain up to 96% of the heat in the hot strip during its passage from the exit of the roughing mill to entry to the finishing mill.
It has proved more difficult, however, to control the lateral temperature profile of the hot material, especially if the rolling mill is arranged to accommodate a wide range of material widths and thicknesses.
In EP 005340, upper heat insulating panels are suspended over the roller table on cantilever supports to allow them to be raised away from the table to avoid damage and to provide access to the table for maintenance. These upper panels are mounted displaceably on their supports, either to slide laterally or to be tiltable, to vary the heat-insulating effect laterally of the roller table. But there is a problem that incorporating such displacement mechanisms for the upper panels significantly increases the inertia of the cantilevered assembly. This entails considerable extra power to ensure that the lifting of the panels can still be performed fast enough to prevent damage from a deformed strip or slab, not to mention the cost penalty of the increased structural support required. It is also proposed in EP 005340 that the bottom panels are laterally slidable but their range of movement is severely limited, in particular by the cantilever support structure and motor drive which must lie close to the side of the roller table.
Another heat shield arrangement proposed in EP 198595 employs displaceable heat shield panels which can be moved with the aim of reducing the clearance between the panels and the hot material. Heat loss from the edges of the material in particular can be reduced to some degree if the side insulating panels can be adjusted close to the material being handled, but this method of control has limited application and is useful primarily for narrower material paths, eg. less than about 800 mm, or where the variation in product width is small.
There therefore remains a need for a cost-effective form of temperature control for hot material on a roller table of a hot-rolling mill that will mitigate the phenomenon of increased edge cooling, especially from relatively thick material, and that can be employed over a wide range of cross-sectional sizes of material.